This invention relates to a photocatalyst having visible light activity and a method of photo-decomposition of a chemical substance by irradiation of light using the photocatalyst.
In recent years photocatalysts are attracting much attention for the production of oxygen and hydrogen from water. Utilization of photocatalysts as an environmental clarification material for removing malodors and for decomposing harmful substances or pollutants in air or water has been also researched extensively. In the case of photo-decomposition of water, an energy-accumulating reactionH2O→H2+ 1/2O2proceeds according to the following photocatalytic mechanism. Thus, when the photocatalyst is irradiated with light having greater than the band gap energy thereof, electrons having strong reducing action and holes having strong oxidizing action are formed. The strong reduction power of the electrons and the strong oxidation power of the holes generated by the optical excitation are utilized for the decomposition of water.
For an industrial scale process for the production of oxygen and hydrogen using a photocatalyst, utilization of sunlight is essential. The intensity of sunlight is the strongest at a wavelength of about 500 nm. A visible light region of sunlight accounts for about 43% of the total solar energy, while only about 5% of the solar energy is attributed to an ultraviolet region. In order to effectively utilize the solar energy, therefore, it is essential to use a photocatalyst having visible light activity.
One well known photocatalyst is titanium oxide. The titanium oxide photocatalyst, however, needs ultraviolet rays having wavelength 400 nm or less as the excitation light and has low visible light activity.